This invention relates to a method of stabilizing poly(arylene ether ketones).
Poly(arylene ether ketones) (PAEK's), in particular para-linked ones, possess many desirable properties, for example, high temperature stability, mechanical strength, and resistance towards common solvents. Two types of PAEK synthesis are known in the art, commonly referred to as electrophilic and nucleophilic synthesis, respectively.
In an electrophilic synthesis, the polymerization step leads to formation of an aryl ketone group, derived from reaction the between an aromatic acid halide (or similarly reactive acyl derivative) and an activated hydrogen atom attached to an aromatic carbon atom, i.e., a hydrogen displaceable under the electrophilic reaction conditions. The monomer system can be (a) phosgene or an aromatic diacid dihalide such as terephthaloyl chloride and a polynuclear aromatic compound containing two activated hydrogen atoms such as 1,4-diphenoxybenzene or (b) a polynuclear aromatic compound containing both an acid halide group and an activated hydrogen atom, such as p-phenoxybenzoyl chloride.
An electrophilic synthesis is sometimes also referred to as a Friedel-Crafts synthesis or polymerization. Typically, it is carried out in a reaction medium comprising the monomer(s), a catalyst such as anhydrous aluminum trichloride, and an inert solvent such as methylene chloride. Because the carbonyl groups of the monomer(s) or other reactant(s) complex with aluminum trichloride and thereby deactivate it, the aluminum trichloride catalyst is generally employed in an amount slightly more than one equivalent for each equivalent of carbonyl groups in the reaction medium. Other metal halides such as ferric chloride can also be employed as the catalyst. The preparation of PAEK'S by Friedel-Crafts polymerization with aluminum chloride as catalyst is disclosed by Bonner, in U.S. Pat. No. 3,065,205 (1962); Goodman, in GB No. 971,227 (1964); and Jansons et al., in U.S. Pat. No. 4,709,007 (1987). Friedel-Crafts polymerization may also be effected in anhydrous hydrogen fluoride-boron trifluoride. See, e.g., Marks, in U.S. Pat. Nos. 3,441,538 (1969); Dahl, 3,953,400 (1976); and Dahl et al, in 3,956,240 (1976).
In a nucleophilic synthesis, the polymerization step leads to the formation of an aryl ether group, derived from the reaction of a phenoxide group with an aryl halide group in which the halide is activated towards nucleophilic displacement. The phenoxide containing monomer may be a bisphenol such as hydroquinone, while the halide containing monomer may be a dihalide such as 4,4,-difluorobenzophenone. Exemplary nucleophilic syntheses are disclosed in Rose et al., U.S. Pat. No. 4,320,224 (1982), and in Attwood et al., Polymer 22, 1096 (1981).
The tendency of their carbonyl groups to form complexes and their crystallinity and insolubility in most solvents make it difficult to isolate PAEK's from the polymerization mixture without their containing catalyst residues and other impurities. Further, such impurities are difficult to remove from the isolated polymer. These impurities are undesirable, because they may adversely affect polymer stability during later use. As part of the regular work-up technique, PAEK's have commonly been dried in vacuo, typically at a temperature between 100.degree. and 160.degree. C., although temperatures up to 220.degree. C. have been used. See, e.g., Marks, Dahl '400, and Dahl '240, cited supra, and Berr, U. S. 3,637,592 (1972).
Special techniques have been proposed for the isolation or post-isolation treatment of PAEK's to improve their properties. Dahl, in U.S. Pat. No. 3,751,398 (1973), discloses a spray drying process in which sulfur dioxide, preferably 90-99% by volume, is admixed into a hydrogen fluoride-boron trifluoride polymerization medium and the mixture is atomized. Maresca, in U.S. Pat. No. 4,611,033 (1986), discloses a stabilization process in which the polymer is treated with a dicarbonyl chelating agent such as pentanedione. Angelo et al., in U.S. Pat. No. 3,767,620 (1973), disclose that certain PAEK's may have xanthydrol end-groups and that the thermal stability of such PAEK's can be improved by reducing the xanthydrol groups to xanthene groups with reductants such as formic acid or triethylsilane. These techniques share a common disadvantage in that they may require the addition of a chemical agent whose own removal from the polymer could be problematic.
We have invented a novel treatment for PAEK's which significantly reduces the impurity levels therein and improves their properties such as melt stability and/or melt viscosity.